The present invention relates to magnetic recording and, more particularly, to a magneto-resistive sensor for reading perpendicularly recorded magnetic media and a system incorporating the same.
Magnetic media in the form of tapes, strips, floppy disks and hard disks are widely used for information storage. Information is recorded by imposing one of two opposing magnetic spin orientations on each of many magnetic domains. In longitudinal recording, the opposing spin orientations are aligned with the plane of the medium, whereas in perpendicular recording the opposing spin orientations are perpendicular to the plane of the medium. As a result, the fields at the media surface above a transition are unipolar in the case of longitudinally recorded media and bipolar in the case of perpendicularly recorded media.
Longitudinal recording has been the easier format to implement and thus is far more prevalent. Perpendicular recording inherently permits greater recording densities and thus is presently subject to intense research and development efforts. To date, however, more is known about optimizing read devices for longitudinal recording. Accordingly, read sensors for longitudinally recorded media have tended to anticipate read sensors for perpendicularly recorded media.
Widely prevalent inductive write heads include elements which can also serve for reading media inductively. Inductive reading is passive in that the unamplified signal output of an inductive sensor derives its energy from the flux levels as well as the rate of change of flux transitions in the medium being read. Consequently, the sensitivity of read sensors is limited by the flux generated by the medium at the sensor. Therefore, inductive read sensors have failed to provide the sensitivity required by the most dense storage media, which tend to provide less flux per unit of stored information.
Active read sensors have been developed in which sensitivity is a function of a sense current and not only of flux strength. Such sensors, which use magneto-resistive elements, can provide greater sensitivity than inductive sensors and so have extended the range of storage densities that can be read reliably. Magneto-resistive sensors have been disclosed for both longitudinally recorded media and for perpendicularly recorded media, for example, in U.S. Pat. No. 4,654,739 to Takahaski et al. A basic magneto-resistive sensor includes a stripe of magnetic material with longitudinal and transverse axis. The stripe is magnetized along the longitudinal, or "easy" axis, which can coincide with the direction for the sense current. The resistance of the stripe can be changed by applying a magnetic field to the stripe so as to rotate its magnetization.
In the absence of a magnetic field, the resistance of the stripe is at a maximum and does not change sensitively in response to magnetic flux changes. Accordingly, the sensor is typically biased by a magnetic field oriented along the "hard" or transverse axis of the stripe. This bias field can be generated by a bias current through a conductor arranged parallel to the magneto-resistive stripe. Preferably, the magnetic bias yields a net magnetization at roughly 45.degree. to both the hard and easy axes. With the magnetization of the stripe so biased, the stripe resistance changes with optimum sensitivity, and linearly, to changes in magnetic flux along the hard axis.
Accordingly, stored information can be read by detecting changes in voltage drops across a magneto-resistive stripe as the medium containing the information is moved relative to the stripe. An advantage of magnetoresistive sensors over inductive sensors is that the sensitivity of the former can be enhanced by increasing the sense current through the stripe. However, there are practical limits on the current which can be supplied through a single stripe. One cannot improve sensitivity by reducing the resistance of the stripe, since that is the variable of interest. One can increase the power supplied to the stripe, but thermal buildup due to heat dissipation in the stripe can disturb the read process and even damage the read head.
The current limitation on single element magneto-resistive sensors has been successfully addressed for longitudinally recorded media by providing co-directional currents through parallel magneto-resistive stripes. A certain design efficiency is attainable using dual magneto-resistive sensors since the sense current for each stripe can serve as the bias current for the other stripe; this eliminates the need for a separate conductor for a bias current. In addition, the voltage outputs of the stripes can be combined differentially to implement common mode rejection of perturbances such as thermal noise.
While other magneto-resistive sensor configurations have been developed successfully the advantages of dual magneto-resistive sensors using parallel currents for reading longitudinally recorded media suggest the use of such sensors for reading perpendicularly recorded media. However, there are significant differences between the media used for perpendicular recording and the media used for longitudinal recording that complicate the use of active sensors, such as single and dual stripe magneto-resistive sensors.
A layer of "hard" or relatively high-coercivity magnetic material is used as information storage media in both formats. A typical perpendicular recording medium employs a "soft" or relatively low-coercivity magnetic underlayer to write the perpendicular orientation of the domains in the hard magnetic layer. This underlayer can amplify magnetic fields induced by currents through active read sensors. These amplified fields can be large enough so that the read process can alter and erase data during the read process. This problem, which occurs with single element magnetoresistive sensors, can be exacerbated when the larger currents available using dual sensors are applied.
Without addressing this problem of using current-bearing sensors with perpendicular media, U.S. Pat. No. 4,589,041 to Voegeli discloses a read sensor for perpendicular recording media (referred to therein as "vertical" recording media) with dual magneto-resistive stripes and co-directional currents. In the disclosed sensor, the stripes are connected electrically in parallel between two terminals. The stripes are spaced apart at a distance which is small relative to the density of the stored data to be sensed. The stripes are mutually biased in opposite directions near respective inflection points in their respective resistance versus magnetic field curves. This is accomplished by current from a constant source attached to a common terminal.
Voegeli's sensor is subject to the read perturbances that generally apply to active read sensors for perpendicularly recorded media. Voegeli neither addresses nor solves the problems caused by these perturbances. In addition, since the stripes are connected "internally", it is difficult to monitor the currents through the individual stripes to ensure that the currents are equal. Unequal currents result in different sensitivities due to different sense currents and bias fields of different strengths. Even were inequalities detectable, there is no provision for balancing the currents.
Consequently, to operate each stripe with optimal sensitivity and linearity requires that the resistances of the stripes be carefully matched across the range of expected read signal flux strengths and operating temperatures. This imposes strict processing constraints on the sensor during fabrication adding to its cost. Furthermore, no provision is made for current inequalities due to device aging.
Thus, heretofore, read sensors for perpendicularly recorded media have been limited to passive inductive sensors with relatively limited sensitivity and active devices with greater sensitivity but which can disturb recorded information during a read operation. What is needed is an active read sensor which minimizes the disturbances at a perpendicularly recorded medium. Preferably, the read sensor would be adaptable for reading longitudinally recorded media as well as perpendicularly recorded media to enhance the operational flexibility of an incorporating information retrieval system.